Reprinted with permission from
Aircraft Maintenance Technology, July 1999
Boosting Your
Knowledge of
Turbocharging
(Part 1 of a 2 part Series)
By Randy Knuteson
A
short 15 years after Orville
and Wilbur made their historic flight at Kitty Hawk,
General Electric entered the
annals of aviation history. In 1918, GE strapped
an exhaust-driven turbocharger to a Liberty
engine and carted it to the top of Pike’s Peak,
CO — elevation 14,000 feet. There, in the crystalline air of the majestic Rockies, they success-
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fully boosted this 350 hp Liberty engine to a
remarkable 356 hp (a normally aspirated engine
would only develop about 62 percent power at
this altitude).
An astounding altitude record of 38,704
feet was achieved three years later by Lt. J.A.
Macready.
This new technology began immediately
experiencing a rapid evolution with the full
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strength of blowers being tested during
WWII. The B-17 and B-29 bombers along
with the P-38 and P-51 fighters were all fitted with turbochargers and controls.
Turbocharging had brought a whirlwind of
change to the ever-broadening horizons of
flight.
Much of the early developments in recip
turbocharging came as a result of demands
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from the commercial industrial diesel engine
market. It wasn’t until the mid-1950s that this
technology was seriously applied to general aviation aircraft engines. It all started with the prototype testing of an AiResearch turbocharger for
the Model 47 Bell helicopter equipped with the
Franklin 6VS-335 engine. Their objective was
not to increase power, but rather to maintain
sea level horsepower at altitude. They succeeded. In the process, a new altitude record for
helicopters of 29,000 feet was achieved.
Shortly afterward, the Franklin Engine
Company entered receivership and in 1961,
Bell ended up with a production helicopter
powered by a Lycoming TVO-435. Coinciding
with these developments were Continental’s
efforts to develop their TSIO-470-B (Cessna
320) and GTSIO-520 (Cessna 411).
Concurrently, efforts were also being made
by TRW and later Rajay to provide 65 STCs to
retrofit engines and airframes for approximately two dozen aircraft. Early OEM installations
of these systems included the factory installed
Rajay in Piper’s Commanche and Twin
Commanche. Other original equipment installations included the Piper Seneca, Turbo
Arrow, Enstrom Helicopter, Mooney 231, and
Aerostars.
Turbo-normalized or groundboosted?
Distilled to the most basic of definitions, a
turbocharger is simply an air pump powered by
the unused heat energy normally wasted out the
exhaust. This “air pump” (or more accurately,
compressor), is capable of supplying the engine
intake manifold with greater than atmospheric
air pressures. A collateral benefit is derived as
the turbo also provides air for the cabin pressurization of certain aircraft.
Some confusion persists as to the difference
between an airplane that is “ground-boosted”
as opposed to one that is “normalized.” Simply
put, turbocharging serves one of two purposes: either it directly increases (boosts) the
power output of the engine, or it assures that
sea level horsepower performance is maintained (turbo-normalized) to higher altitudes,
thereby increasing the plane’s potential service
ceiling.
A “normalized” turbo installation like the
Rajay system in no way increases the normal
engine RPMs, loads, or BMEP limits already
established as safe for the engine. Instead, it
merely assures that sea level performance is
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Percentage of HP Available At Altitude
100%
90%
Turbocharge
80%
70%
60%
No
50%
n-T
urb
oc
ha
40%
rge
30%
20%
10%
25 50 75 100
150
200
250
300
350
Altitude (X100)
* Percentage based on full throttle, maximum RPM, standard day temp.
This graph illustrates the dramatic disparity between turbocharged
and non-turbocharged aircraft engines.
to bring these temperatures down and recapture some of this power loss while suppressing
detonation.
An engine relying on manifold pressures
greater than ambient on a standard day is considered a “boosted” system. These engines
require manifold pressures ranging from 31.0 to
45.0 inches HgA. Installations incorporating
intercoolers demand an additional 2 to 3 inches HgA to compensate for the pressure loss in
air flow through the intercooler. Common
installations include TCM’s TSIO-520s in the
Cessna 210 and Lycoming’s TIO 540 and 541
installed in Navajos, Turbo Aztecs, and Dukes,
to name a few. These engines require reduced
compression ratios (to provide wider detonation margins) since they produce more than
normal sea level manifold pressures while in
the take-off and climb configuration.
Basically, the ultimate goal of turbocharging
is to gain more power or to increase the
efficiency of the engine without enlargdefinitions, ing the powerplant.
maintained at altitude without the customary
diminishment of power. An engine that sustains but does not exceed 29.5 inches of manifold pressure at altitude is said to be normalized. “Critical altitude” is that point above
which the turbocharger can no longer maintain
maximum rated manifold pressure. However,
just because an engine maintains 29.5 inches of
MAP and redline rpm, does not necessarily
mean it is developing sea level power.
Depending on the application, compressor discharge air at critical altitude may be as hot as
250 to 300 degrees Fahrenheit. An increase in
induction air temperatures of 6 to 10 degrees
Fahrenheit decreases horsepower by roughly 1
percent. So, an airplane with a critical altitude
of 25,000 feet may be producing only 80 percent power even though sea level manifold
pressure is indicated at that altitude. An intercooler serves the purpose of a heat exchanger
“
Distilled to the most basic of
a turbocharger is simply an air pump
Design differences
powered by the unused heat energy
Turbochargers manufactured by Rajay
and Garrett are very similar. Perhaps the
most striking difference is their compara-
normally wasted out the exhaust.
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tive size. Rajay units weigh 12 pounds while the
Garrett turbos weigh from 15 to 43 pounds.
These radical size variances are associated with
engine size and applications. For instance, the
TAO4 Garrett and the Rajay Turbos are typically installed in 180-230 hp engines. Compressor
diameter in these models vary from 2.755 inches to 3.0 inches. In the Aerostar, twin turbos of
this size are used. Both TCM’s earlier model
310P in the Malibu, and the Lycoming powered
Mirage, also use this dual turbo arrangement.
The TEO6 turbocharger is used to boost the performance of the 520s and 540s in the 275 to 350
horsepower category. While the 340s, 414s, and
Navajos rely on a larger compressor wheel of
the TH08 turbocharger to provide the additional bleed air for cabin pressurization.
Internally, the bearing design of the Rajay
is that of a “semi-floating” journal bearing.
While the Garrett design incorporates dual
bearings that turn at half of the turbine wheel
speed. Instead of a ductile cast-iron bearing
housing, Rajay’s housing is manufactured from
aluminum. Both turbo lines depend on the
Rajay (formerly Garrett AiResearch) valves and
controllers to monitor turbo discharge and to
determine manifold pressure. There are
exceptions; however, most noticeably TCM’s
“fixed” wastegate in their Seneca and Turbo
Arrow models, and the Rajay manual wastegates and controllers.
Increased efficiency in a rarified
atmosphere
At sea level, the atmosphere in which we
live and breathe is continually under a pressure of about 29.92 inches of mercury (Hg).
At 1,000 feet, this “free air” drops in pressure
to about 28.86 inches Hg. Air becomes progressively less dense at all altitudes above
sea level. Because of this, all naturally aspirated engines experience a reduction of fullthrottle, sea level power output as they
increasingly gain altitude. On a “standard
day,” atmospheric pressure at 10,000 feet
hovers at only 20.5inches Hg. In these conditions, a naturally aspirated engine is
unable to sustain full power performance
due to the lack of air density at these higher
altitudes. Without the aid of a turbocharger,
a loss of 3 to 4 percent of power or approximately 1 inch of MAP per 1,000 foot gain in
altitude seems to be the rule.
With the fuel to air ratio remaining constant, the amount of power developed by an
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Surge Line
Area of Peak
Efficiency
RPMS
Turbocharger compressor performance map for Orenda OE600A
engine — 600 hp at sea level take-off. This compressor can provide pressurized
airflow for 500 engine horsepower at altitudes in excess of 25,000 feet. The left side
of the map is referred to as the “Surge Area” where pressure and flow is unstable.
The center “Island” represents the area of peak efficiency. As a rule, the broader
the range of the compressor, the lower the peak efficiency.
engine is directly proportionate to the mass of
air being pumped into the engine. On a nonturbocharged engine, an increase in airflow is
achieved by changing the throttle angle to
increase MAP, or prop-pitch to increase RPM.
The atmospheric limitations of density altitudes coupled with the mechanical limitations
of the engine and propeller pose as restrictions to engine speeds. Some means of forcefeeding the engine additional air is required to
overcome these limitations.
Re-claiming wasted energy
As much as one-half the total heat energy
from the engine is lost through the exhaust as
a natural by-product of the combustion
process. A portion of this heat energy is recovered by harnessing the hot expanding exhaust
gases and re-routing them through the turbocharger. These gases enter a radial inflow
turbine housing, striking the blades of the
Inconel turbine wheel and exiting the turbine
housing outlet. This flow of gas provides the
necessary thrust that enables the turbine wheel
to rotate at high speeds. Since these exhaust
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gases are normally wasted, little power is
robbed from the engine to drive the turbine. A
centrifugal compressor is connected to the turbine wheel by a common shaft, enabling it to
rotate at the same speed as the turbine. The
impeller draws in filtered air, compresses and
delivers it to the cylinders where each pound of
fuel is mixed with about 14.8 pounds of air
(stoichiometric or peak EGT). In reality, however, proper engine management dictates that
most engines be operated 100 to 125 degrees
rich of peak EGT, thereby altering this ratio.
Wheel configuration and size, housing size, and
shaft speed all determine the pressure and volume of air delivered to the engine.
Reams of data compiled by the engine
manufacturers outline their desired parameters
for the turbochargers and control systems to
be used on their reciprocating engines. Design
engineers then analyze this information, basing performance predictions on expected temperature and pressure changes in the manifold, the induction and exhaust ducting, as
well as the pressure drop across the throttle
and through the intercooler. The placement
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and orientation of the air intake, the exhaust
by-pass valve, and cabin air-bleed must be
factored in. The parameters of BHP (brake
horsepower), turbine inlet temps, bypass flow
and compressor discharge temps are only a
few of the elements plotted against altitude for
selected manifold pressures.
Finally, flight-testing verifies or invalidates
the assumptions of expected engine performance. Continued analysis is made, errors are
corrected, assumptions are cross-checked, and
the accuracy of the data is refined with a high
probability of obtaining the desired aircraft
performance.
The goal is to use little or none of the
engine’s horsepower to drive the turbocharger. Consequently, when proper engine installation and matching is done, mechanical loads
imposed on the engine to power the turbo are
minute. Most of the work required to drive the
turbine is recovered from the exhaust gases.
Otherwise, if the turbocharger was not in the
system, this portion of the gas energy would
be lost with discharged exhaust gases.
Maintenance of the turbocharger
The turbocharger itself is subjected to an
extremely hostile environment. Turbine inlet
temperatures reach a scorching 1,650 degrees.
Some are hotter still. TCM’s liquid-cooled
Voyager is rated at 1,750 degrees continuous
with the capability of 1,800 degrees for 30 seconds while establishing peak EGTs. Turbine
speeds range from 0 to 120,000 rpm. The T36
in the Malibus and the Lancair 4P is capable of
125,000 at 1,650 F. That’s a screaming 2,083
revolutions per second! Pulsing exhaust gases,
engine vibration and temperature variations all
add to this hellish mix.
The turbo assembly is made of the
strongest of alloys available to withstand these
rigorous conditions. However, these tortuous
duty cycles can shorten the life of a turbo if
proper maintenance is not performed. It is of
the utmost importance to follow the maintenance and inspection criteria spelled out in
the applicable service bulletins and airworthiness directives.
Oil contamination, oil supply problems,
FOD damage, and abrupt temperature
changes are the chief contributors to premature turbocharger failures. There can not be
enough emphasis placed on the importance of
keeping the oil clean. Remember that the oil
used to lubricate and cool the turbocharger is
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A classic example of turbine wheel damage caused by a piece of
exhaust valve face. This demonstrates the importance of examining
the condition of the turbine side of the turbocharger whenever
cylinder or exhaust system maintenance is required.
the engine oil. At high rpms, dirty oil that
results from combustion by-products and the
carbon residue from coking can dramatically
shorten the life of a turbo. Although engine
manufacturers recommend oil changes at 50hour intervals, many engine overhaul shops
suggest a more conservative interval of 25 to
35 hours with turbocharged engines.
Restrictions in the oil supply to the turbo
result in a reduction of oil flow and subsequent overheating of the bearing(s) or center
housing. Oil starvation can be caused by bad
gaskets, restricted oil flow or improperly positioned orificed T-fittings. A classic example is
the restricted T-fitting in the oil inlet line on
Cessna’s 210, 206, 207, and 337s. The restricted side of this fitting is meant to feed the oil
pressure gauge, not the turbo. The turbo side
of this fitting is unrestricted. Unfortunately, it
occasionally gets plumbed incorrectly.
At all 100-hour inspections, remove the
induction air supply duct to the compressor
and the separate the exhaust outlet ducting
from the turbine side. Check the compressor
and turbine wheel blades for potential foreign
object damage. Also, examine the outer tips of
the blades and adjacent housing surfaces for
any evidence of drag or rubbing.
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Turn the wheels by hand while exerting an
end and side-load. There should be no rubbing or binding of the wheels against the
housings and they should be free to rotate. Be
certain to check the turbine housing for cracks
and the security of the exhaust housing bolts,
the lock tabs and the condition of the “V”
band clamps. Special attention should be
given to the manufacturer’s recommendations
for proper “V” band installation procedures
and appropriate torque values.
A preventative maintenance procedure that
should be practiced universally is to allow the
engine to sufficiently warm up before applying full power, and to avoid pulling power
abruptly. It’s also advisable to allow the
engine to idle an additional three to five minutes prior to shutting it down. This allows the
turbo to cool down and equalize temperatures
before going to idle cut-off. These simple
measures will reduce the possibility of oil
residue coking in the hot turbine housing and
should prolong the life of the turbo.
It’s only as you consider the operational
demands and the exacting details that factor
into the analytical performance predictions in
designing these systems that you gain a true
appreciation for aircraft turbocharging.
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Reprinted with permission from
Aircraft Maintenance Technology, October 1999
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Boosting Your Knowledge
of Turbocharging
(Part 2 of a 2 part Series - Values and Controllers)
By Randy Knuteson
A
ll normally aspirated aircraft
engines gain altitude at the
expense of horsepower.
Approximately three percent of horsepower is lost
or traded for every 1,000 feet of altitude
gained; as altitude increases, performance
wanes. The obvious advantage of any turbocharged aircraft is its ability to compensate for this loss and provide maximum
rated power at altitude. This advantage
gives the pilot the ability to over-climb
inclement weather, achieve optimum altitudes and attain maximum speeds with relative ease. To fly high and fast in a pressurized cabin while experiencing smooth air
and favorable winds is very desirable.
Reduced drag, increased range, and
improved fuel economy are just a few of the
benefits turbocharging offers.
In our previous discussion (July issue of
AMT), we focused solely on the turbocharger
itself — that component which at altitude
supplies additional power to otherwise anemic performing engines. We determined that
pulsing exhaust gases provide the requisite
force to drive the turbine wheel and consequently, the compressor, to speeds of up to
2,000 revolutions per second. The end result
— exhaust energy is converted to manifold
pressure. We also discovered that the turbocharger is subjected to the extremes of
temperatures and duty cycles and that consistent oil changes, cool-down practices and
maintenance are essential to its continued
performance, health and operation.
Turbochargers are indeed a remarkable
feat of engineering ingenuity. And when coupled with their supporting wastegates, controllers, relief valves, oil lines and piping they
can seem both confusing and intimidating.
Occasionally, even the seasoned mechanic
becomes frustrated to the point of literally
“throwing in the wrench” on these systems.
However, armed with just a bit of
knowledge, charge-air systems can
. . . turbochargers require some means of be reduced from extremely daunting
to simply challenging. Perhaps the
control. It becomes obvious that each turbo
following information will equip you
to feel up to the challenge as we
control system chosen for a specific airplane
attempt to clarify some of the annoying and at times confusing nuances
has its own unique peculiarities.
of turbocharging systems. Our dis-
“
”
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cussion will focus on the individual components coupled with an explanation as to how
they interrelate to each other.
Pressures to Perform
As their names suggest, Valves and
Controllers both regulate and control turbocharger discharge pressure. This is an
exacting science due to the fact that there is
a disproportionate air/fuel ratio that must be
tailored to the ever-changing demands of the
engine as well as atmospheric density
changes. For these reasons, a means of managing this forced air-flow to the cylinders is
necessary to prevent the onset of detonation
or overboost. The control portion of the turbocharging “system” was designed for this
purpose. Valves and controllers provide the
desired flight envelop, while keeping engine
intake manifold temperatures and exhaust
manifold pressures as low as possible.
Deck Pressure and Manifold Pressure
A basic knowledge of the air pressures associated with turbocharging is necessary for a
more thorough understanding of these control
systems. Air upstream of the throttle plate is
often referred to as “Upper Deck Pressure.”
Deck pressure is measured between the compressor discharge and the inlet to the fuel injector or carburetor. Pressure downstream of the
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throttle is referred to as “Manifold Pressure” and
is referenced between the throttle plate and the
cylinder intake. A pressure drop occurs across
the throttle plate and is dependent upon the
position of the throttle angle. Deck pressure always exceeds manifold pressure. At wide-open
throttle, the air pressure drop across the throttle
plate is at its minimum (as little as 1/4 in. HgA).
And conversely, as the throttle is closed, this
pressure drop increases. These pressures
change both in response to throttle movements
and variations in air density and temperatures.
Air density is a function of both pressure
and temperature. Therefore, air temperatures
influence the upper deck and manifold pressures. As you recall, as air is compressed, it
increases in temperature, resulting in a reduction of air density and consequently a loss of
engine power. A natural reduction of air density also occurs at higher altitudes and at temperatures above standard, which further exacerbates this problem. The compressor is forced
to work harder to feed a sufficient amount of
air to the cylinders. Excessive heat becomes the
undesirable by-product of this exchange.
Generally, there is a loss of one percent of
horsepower for every 6 to 10 degrees increase
in induction air temperature. Remember,
engine power is influenced not only by rpm
and manifold pressure, but also by induction
air temperature. In certain installations an intercooler acts as a heat exchanger to cool down
this discharge air and recapture lost power.
The need to control
Intake air pressures and temperatures
must be controlled since they have such a
direct influence on engine performance. The
Controller’s purpose is twofold: to sense
changes in the upper deck and intake mani-
fold pressures and to keep engine power
constant as ambient conditions vary. Some
engines rely on more than one controller to
perform this task. Controllers may be found
in multiples or combined in a common housing. Regardless of the arrangement, they
operate in parallel, even while responding to
differing stimuli. Depending upon their installation, controllers may be plumbed directly
into the upper deck air-stream or remotely
mounted and referenced by means of hoses.
Automatic controllers
The automatic controller monitors deck,
manifold, and/or ambient pressures by
means of a pressure differential across the
diaphragm or by using a bellows calibrated to
a predetermined absolute pressure. As the
controller senses pressure changes, it seeks to
modulate turbo output by regulating the
amount of oil allowed to bleed past a poppet
valve also housed within the controller. The
movements of the diaphragm or the expansion and contraction of the bellows within
the controller alters the position of this oil
metering poppet valve in relation to a seat. As
this poppet closes, oil flow back to the sump
is restricted. Oil begins to dam upstream and
exert a force against a piston in the wastegate
actuator. This piston is mechanically linked to
the wastegate valve and begins to close the
valve. As the controller poppet moves, it
becomes an adjustable orifice regulating the
buildup of oil pressure that is exerted against
a piston in the wastegate actuator. By altering
the position of the wastegate valve, it controls
the amount of exhaust gases either routed to
the turbo or diverted out the exhaust, thus
maintaining a pre-selected manifold pressure
setting (see diagram 1, following page). In this
manner it schedules the appropriate amount
of air to the engine at any given altitude or
power setting.
The controller(s) could be appropriately
thought of as the brains or command center
for the entire charge-air system. As engine
power, speed, or altitude is changed, the controller constantly strives for equilibrium within the system. The exhaust bypass valve
(wastegate assembly) simply acts as a slave to
the dictates of the controller. Engine oil
becomes the muscle of the system, providing
the power to actuate the movements of the
wastegate assembly.
Comparative graph illustrating the
advantage of turbocharging over
naturally aspirated engines. This
chart also demonstrates the differences
found between the three distinct
wastegate arrangements.
Design differences
Lycoming engines utilize four basic styles of
controllers: the Differential, the Density, the
Variable Pressure and the Slope Controllers.
The density controller is the only controller
capable of sensing changes in temperature. It
relies on a bellows charged with dry nitrogen
to accomplish this purpose. Density controllers
are found in the Piper Navajo and Chieftain
and a handful of helicopter applications. In the
Piper, the density controller modulates the
wastegate movement at wide open throttle
while a differential controller keeps deck pressures from exceeding manifold pressures by
more than a specified amount at part throttle
settings. Density controllers are extremely sensitive to in-field adjustments. A 1/16th turn of
the adjustment screw will result in a 2-in.
change in manifold pressures. A thermocouple
probe referenced to deck temperature is
required when setting up these systems (see
Lycoming S.I. No. 1187J).
Raytheon Aircraft’s turbocharged Barons,
Bonanzas, and Dukes depend on a Variable
Absolute Pressure Controller. The VAPC has a
bellows that extends into the upper deck air
stream, sensing deck pressure and comparing it
to a referenced absolute pressure. What makes
this controller “variable” is that it is linked directly to the engine throttle. Through a system of
cams and followers, it adjusts a moveable poppet seat and accordingly achieves the optimum
deck pressure for a given throttle setting.
Examples of Valves and Controllers
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Exhaust bypass valves: butterfly, poppet and fixed. Note that Continental’s
“fixed” wastegate system relies on an adjustment screw in the turbine bypass
pipe (right). Ground adjustments are made by loosening the lock nut and
turning the screw into or out of the wastegate bypass pipe.
Continental’s turbocharged engines rely on
absolute, variable absolute, pressure ratio,
dual, or slope controllers. Dual controllers are
a hybrid combination of absolute controllers
coupled with a rate or ratio controller. These
units are housed in a common body and were
most frequently used in the older Cessna 200300-, and 400-series engines. TCM equipped
Piper Malibus, the TSIO-550 equipped experimental Lancair IV, Piper’s new PA-32 and
Cessna’s new 206’s all incorporate yet another style of controller — the sloped controller.
The sloped controller incorporates the functions of an absolute pressure controller and a
differential controller in a single housing.
Acting much like the density and differential
arrangement in the Navajo, the sloped controller maintains the rated deck pressure at
wide-open throttle, and a reduced deck pressure at part throttle, and looks for discrepancies in the differential between deck and manifold pressures. If either pressures rise above
a pre-determined value for a given throttle setting, the sloped controller opens the exhaust
bypass valve, thereby lowering compressor
speed and output. However, unlike the density controller, it does not correct for temperature excursions.
Manual control systems
Not all turbocharging systems are automatically actuated. Instead, systems like those
found in the Turbo Arrow and the Mooney 231
have a simple, built-in exhaust leak upstream
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of the turbo that allows a fraction
Diagram 1: Schematic diagram showing the functional
of the exhaust gases to be dumped
operation of the type of turbocharging system found
commonly on Raytheon’s Duke, the Cessna P210, 340,
overboard and the remainder to be
414, 421, Piper’s Malibu and pressurized Navajo.
routed through the turbine. This
“fixed” wastegate arrangement has
inches of Hg during throttle advancement.
also been successfully used in the Seneca II-IV.
To compensate, pilots were instructed to
Other systems rely on manual inputs from the
slowly advance the throttle to 32-in., wait
pilot. As power begins to decrease at higher
for a few seconds, and then continue to the
altitudes with the throttle in the wide-open
rated 35-in. of MAP. This surge in overboost
position, the pilot then incrementally adjusts
was dramatically reduced to 2 to 3 inches of
the wastegate toward closure utilizing a sepaHg with the development of the GTSIO-520
rate vernier wastegate control cable. Or, as in
engines. By modifying the bore size of the
the case of Cessna’s Turbo Skylane and Piper’s
wastegate actuator, designers found they
Turbo Saratoga, some installations have the
could minimize the time it took for oil diswastegate linked directly to the throttle. As the
placement and wastegate actuation.
wastegate valve closes, backpressure forces
Safeguards are required to prevent the
additional exhaust gases through the turbine
overboosting of an aircraft engine.
wheel assembly and speeds up the compresOverboosting occurs whenever maximum
sor. This series of events results in an increase
rated manifold pressures are exceeded.
in engine manifold air pressure and a resulting
Properly adjusted controllers and proper
increase in power. The pilot becomes the “conthrottle management serve to preclude the
troller” of these systems as he systematically
onset of overboost. However, despite these
monitors and regulates manifold pressures
measures, inadvertent overboost still hapwhile ascending/descending in altitude. These
pens. An incidence of overboost can take
simplified manual systems reduce both cost
place when the engine oil has not yet
and complexity, albeit at the expense of
reached operating temps and the throttle is
increased pilot workload. This type of arrangerapidly advanced for takeoff or when a pilot
ment is often used in aftermarket turbo-noron short final quickly advances the throttle
malizing kits.
for a go-around and the prop governor lags
Overshoot/Overboost
or the wastegate sticks. Overboost can also
During the early stages in the developoccur at full throttle and below critical altiment of these systems, problems arose with
tude when exhaust gases expelled are more
momentary overboost of as much as 6 to 10
than capable of driving an uncontrolled
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mally held in the closed position by a
spring/bellows combination. It is set to
off-seat at approximately two inches
above
normally
rated
MAP.
Continental Motors compares the
operation of this spring-loaded valve
to a safety valve in a steam boiler. As
excessive pressure builds, the valve
opens and stays open until the manifold pressure again falls within acceptTwo styles of PRVs. The valve on the right has an
external non-damped spring which is a separate
able limits. The aneroid-style bellows
part surrounding the bellows. The valve on the left
allows the valve to “crack” open at a
is designed for high-vibration environments. The
specified pressure regardless of the
spring is encased within the bellows assembly that is
operational altitude of the engine.
partially oil-filled to provide vibration damping.
In the Mooney 231 (TSIO-360compressor to engine damaging speeds.
GB/LB), and the Piper Turbo Arrow and
What differentiates between Overshoot
Senecas (TSIO-360-E/EB/F/ FB/KB), the
and Overboost? Answer: Duration, or the
Absolute Pressure Relief Valve functions as a
length of time manifold pressures exceed
turbocharger control/boost limiter. These
their maximum rate. “Overshoot” occurs
engines incorporate a “fixed” wastegate
when manifold pressure increases above
arrangement and do not rely on other servo
maximum rate by one or two inches, and
control systems. In this design, the PRV is set
then returns immediately to the maximum
to vent any compressor discharge pressures
rated. Lycoming states that if overshoot
exceeding one inch Hg above rated manidoes not exceed two inches and three secfold pressure. This “crack” pressure increasonds duration, it may be disregarded. TCM
es as a function of altitude, thereby mainsays, “If the amount of overboost is from
taining a constant manifold pressure as relief
three to six inches, the system should be
flow past the valve head varies. This “modchecked immediately for necessary adjustulating” valve successfully compensates for
ments or replacement of the malfunctioning
variations in compressor discharge air-flow.
component.” As defined by Textron
Lycoming, “momentary overspeed” is “an
increase of no more than ten percent of
rated engine RPM for a period not exceeding three seconds.”
A table for computing overspeed by
engine model is included in Lycoming
SB#369I. Stringent inspection criteria are
specified for engines subjected to inadvertent overspeed. These procedures include a
thorough examination of the cylinders,
Kelly Aerospace Power Systems is in the
valves, guides, seats, counterweights, etc.
How do you spell relief?…P.R.V.
The Pilots’ responsibility is to ensure that
manifold pressures stay within their prescribed limits. But, humans are known to be
fallible. With this in mind, thoughtful engineers sought to incorporate a secondary
safety feature known as an absolute Pressure
Relief Valve (“pop-off valve”). This valve is
strategically positioned between the compressor outlet and the fuel injector/carburetor to prevent engine-damaging surges in
manifold pressure. The valve head is nor-
A i r c r a f t
process of developing the next generation
of turbocharger control systems. This state
of the art electronic controller/wastegate
configuration will provide a direct replacement for the current hydraulic-style controllers. The actuating motor will be retrofittable to existing models. This system is
capable of operating in temperatures from 50 to 300 degrees F with humidity ranging
from 0 to 100 percent at pressure altitudes
up to 30,000 feet. Both controller and
wastegate will be FAA certified for usage in
single and multi-engine, pressurized and
non-pressurized aircraft. Cost will be comparable to hydraulically activated systems.
M a i n t e n a n c e
Kelly Aerospace Power Systems will be offering
its Aircraft Turbocharger Valve and Control
Troubleshooting Reference Guide and its
newly revised and updated Overhaul Manual
for Valves and Controllers by year end.
A Compromise Between Cost and
Performance
As noted earlier, turbochargers require
some means of control. It becomes obvious
that each turbo control system chosen for a
specific airplane has its own unique peculiarities. The final choice of system depends
mainly on the desired altitude characteristics of that specific airplane. Engine horsepower, turbocharger speed limitations,
cabin pressurization, and cost all become
critical deciding factors in this equation.
While a complete integration of the control
system to the turbocharger is desired, occasionally the choice becomes a compromise
between cost considerations and performance characteristics.
Admittedly, this information only provides a cursory understanding of these systems. A more complete appreciation of turbocharger control systems can be had by
studying the RAJAY (previously GARRETT)
Turbochargers, Valves and Controls
Troubleshooting Reference Guide. This brief
but complete guide is available on request
from Kelly Aerospace Power Systems. The
troubleshooting guide along with newly
updated Valve/Controller Overhaul Manual
will be made available by year’s end.
Randy Knuteson is Director
of Product Support for
Kelly Aerospace Power Systems
in Montgomery, AL.
KA031003
T e c h n o l o g y
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